The technological operation of preparing pressure-sensitive adhesives (PSAs) is subject to continual ongoing development. In the industry, hotmelt processes with solventless coating technology are of growing importance in the preparation of PSAs. This development is being pressed further by ever more stringent environmental regulations and increasing prices for solvents. Consequently there is a desire to eliminate solvents as far as possible from the manufacturing operation for PSA tapes. The introduction of the hotmelt technology is imposing growing requirements on the adhesives. Acrylate PSAs in particular are a subject of very intensive investigation aimed at improvements. For high-level industrial applications, polyacrylates are preferred on account of their transparency and weathering stability. As well as these advantages, however, the acrylate PSAs must also meet stringent requirements in respect of shear strength. This is achieved by means of polyacrylates of high molecular weight and high polarity, with subsequent efficient crosslinking. Efficient crosslinking is obtained most easily by means of metal chelates, which at elevated temperatures react with carboxylic acid functions and so crosslink the acrylate PSA. This method is state of the art for solventborne PSAs.
For hotmelt operations preference is given to electron beam curing (EB curing or EBC) since it enables even fairly thick films to be crosslinked. Electron beam curing requires no thermal energy, and crosslinking takes place in a relatively short time.
The first EB-curing polyacrylate hotmelts were described in DE 21 31 059 A1. Further EB-curing hotmelts were developed in JP 05017726. U.S. Pat. No. 5,194,455 described the addition of N-tert-butylacrylamide monomer in order to force forward the EB curing.
A general disadvantage of EBC is the backing damage. The electron beams penetrate not only the adhesive but also the backing material or the release paper. This results in damage, which is manifested in instances of discoloration or in high unwind forces for the adhesive tape. The need is therefore for a hotmelt PSA crosslinking method which is both gentle to the backing and efficient.
For some time now UV-crosslinkable hotmelt PSAs have been available commercially under the trade name acResin®. These compositions, by virtue of their relatively low weight-average molecular weight (Mw of about 200 000-300 000 g/mol), have very good coating qualities and can be crosslinked subsequently by means of UV irradiation. Disadvantages, however, are the inhomogeneity of crosslinking because of a dose profile, low efficiency in the case of resin-modified acrylate compositions, and a limitation of coat thickness to well below 100 μm, thereby ruling out their use for substantial areas of industrial adhesive tapes.
It is also proposed that reactive groups be protected and then liberated only after the coating operation, by means of a mechanism in the presence of crosslinkers such as polyfunctional isocyanates or epoxides, and hence that crosslinking be carried out. An example of this kind of crosslinking, carried out by means of UV initiation with the aid of a photoacid generator, is the application EP 1 127 907 A2. A disadvantage of this process is the liberation of the protective group: in this specific case, the liberation of gaseous isobutene.
Direct thermal crosslinking of acrylate hotmelt compositions containing NCO-reactive groups is described in EP 0 752 435 A1. The blocking-agent-free isocyanates used, particularly sterically hindered and dimerized isocyanates, require very drastic crosslinking conditions, and so a rational industrial implementation is not possible.
Under the conditions such as those which prevail when carrying out processing from the melt, the procedure EP 0 752 435 A1 describes leads to a rapid, relatively far-reaching crosslinking, so that processing of the composition is difficult, particularly with regard to the coating of backing materials. In particular it is not possible to obtain very homogeneous adhesive layers of the kind needed for numerous technical applications of adhesive tapes.
Also state of the art is the use of blocked isocyanates. A disadvantage of this concept is the liberation of blocking groups or fragments, which have an adverse effect on the technical adhesive properties. An example is U.S. Pat. No. 4,524,104. It describes acrylate hotmelt PSAs which can be crosslinked with blocked polyisocyanates together with cycloamidines or salts thereof as catalyst. With this system, one factor is that the catalyst required, but in particular the resultant HCN, phenol, caprolactam or the like, can severely impair the product's properties. Another factor affecting this concept is the drastic conditions required to liberate the reactive groups. No significant deployment of the product has yet been disclosed, and such deployment would anyway seem unattractive.